Between basically nothing and way to much depending on the amount you transfer, the method you use and the country you live in

Proton is 10 years old, so it’s hardly a startup anymore. It was crowdfunded and is mostly led/owned by cern scientists (inventors of the www) and also financed/owned by a swiss non profit focused on helping startups, which in turn is financed by the swiss government (geneva to be exact)

Skiff was founded in the US by americans.

Do you see the difference?

An email provider can encrypt your data so they can’t read it. But they can’t prove that they did that. So just like any other online service you have to trust them or not rely on their encryption.